Touch determination with signal enhancement

ABSTRACT

Multi-touch sensitivity is enabled using a touch-sensitive apparatus comprising a panel for conducting signals from a plurality of incoupling points to a plurality of outcoupling points, thereby defining detection lines between pairs of incoupling and outcoupling points. Signal generators coupled to the incoupling points generate the signals, and signal detectors coupled to the outcoupling points generate an output signal indicative of one or more touches on the surface portion. A signal processor obtains the output signal which, if converted into a set of data samples of a given input format, enables a predetermined reconstruction algorithm to determine an interaction pattern on the surface portion. The signal processor generates, based on the output signal, a modified set of data samples in the given input format; and operates the predetermined reconstruction algorithm on the modified set of data samples so as to determine a modified interaction pattern on the surface portion.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of Swedish patent applicationNo. 1051323-2, filed on Dec. 15, 2010, and U.S. provisional applicationNo. 61/423,273, filed on Dec. 15, 2010, which are incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to touch sensing systems and dataprocessing techniques in relation to such systems.

BACKGROUND ART

Touch sensing systems (“touch systems”) are in widespread use in avariety of applications. Typically, the touch sensing systems areactuated by a touch object such as a finger or stylus, either in directcontact, or through proximity (i.e. without contact), with a touchsurface. Touch sensing systems are for example used as touch pads oflaptop computers, in control panels, and as overlays to displays on e.g.hand held devices, such as mobile telephones. A touch panel that isoverlaid on or integrated in a display is also denoted a “touch screen”.Many other applications are known in the art.

To an increasing extent, touch systems are designed to be able to detecttwo or more touches simultaneously, this capability often being referredto as “multi-touch” in the art.

There are numerous known techniques for providing multi-touchsensitivity, e.g. by using cameras to capture light scattered off thepoint(s) of touch on a touch panel, or by incorporating resistive wiregrids, capacitive sensors, strain gauges, etc into a touch panel.

WO2010/064983 and WO2010/06882 disclose another type of multi-touchsystem which is based on frustrated total internal reflection (FTIR).Light sheets are coupled into a panel to propagate inside the panel bytotal internal reflection. When an object comes into contact with atouch surface of the panel, two or more light sheets will be locallyattenuated at the point of touch. Arrays of light sensors are locatedaround the perimeter of the panel to detect the received light for eachlight sheet. Data from the light sensors may be processed intologarithmic transmission values, which are input into an imagereconstruction algorithm that generates a two-dimensional distributionof attenuation values over the touch surface. This enables determinationof shape, position and size of multiple touches.

A similar type of multi-touch system is disclosed in WO2009/077962.

As the availability of multi-touch systems increases, and inparticularly as these systems are made available in a wide range ofsizes and enabling an increased number of simultaneous touches, it canbe foreseen that software applications with advanced user interactionwill be developed to be run on devices with these types of touchsystems. For example, a user may be allowed to enter advancedmulti-touch gestures or control commands, in which fingers on one orboth hands are dragged across a touch surface, and it may be possiblefor several users to work concurrently on the touch surface, either indifferent application windows, or in a collaborative application window.

Irrespective of sensor technology, the touches need to be detectedagainst a background of measurement noise and other interferences, e.g.originating from ambient light, fingerprints and other types of smear onthe touch surface, vibrations, detection artifacts, etc. The influenceof measurement noise and interferences may vary not only over time butalso within the touch surface, making it difficult to properly detectthe touches on the touch surface at all times. Furthermore, the degreeof interaction between a touching object and the touch surface may varyboth over time and between different objects. For example, theinteraction may depend on if an object is tapped, dragged or held in afixed position onto the touch surface. Different objects may yielddifferent degree of interaction, e.g. the degree of interaction may varybetween fingers of a user, and even more so between the fingers ofdifferent users.

The combination of several touches, complex gestures as well as temporaland spatial variations in degree of interaction, background, and noisewill make the identification of touches a more demanding task. The userexperience will be greatly hampered if, e.g., an ongoing gesture on atouch screen is interrupted by the system failing to detect certaintouches during the gesture.

SUMMARY

It is an object of the invention to at least partly overcome one or morelimitations of the prior art.

In view of the foregoing, one object is to enable a consistent userexperience when interacting with a multi-touch system.

This and other objects, which may appear from the description below, areat least partly achieved by means of a method of enabling touchdetermination, a computer program product, a device for enabling touchdetermination, and touch-sensitive apparatuses according to theindependent claims, embodiments thereof being defined by the dependentclaims.

A first aspect of the invention is a method of enabling touchdetermination based on an output signal from a touch-sensitiveapparatus. The touch-sensitive apparatus comprises a panel configured toconduct signals from a plurality of incoupling points to a plurality ofoutcoupling points, thereby defining detection lines that extend acrossa surface portion of the panel between pairs of incoupling andoutcoupling points, at least one signal generator coupled to theincoupling points to generate the signals, and at least one signaldetector coupled to the outcoupling points to generate the output signalwhich is indicative of one or more touches present on the surfaceportion. The method comprises: obtaining the output signal which, ifconverted into a set of data samples of a given input format, enables apredetermined reconstruction algorithm to determine an actualinteraction pattern on said surface portion; generating, based on theoutput signal, a modified set of data samples in said given inputformat; and operating the predetermined reconstruction algorithm on themodified set of data samples so as to determine a modified interactionpattern on said surface portion.

The first aspect may be used to increase the ability to detect multipletouches on the surface portion based on the output signal, since themodified set of data samples may be generated so as to facilitate touchdata extraction in the resulting interaction pattern, e.g. by enhancingcertain desired features in the interaction pattern. Such enhancementmay e.g. aim at enhancing the appearance of weakly interacting touchescompared to strongly interacting touches and/or reconstruction artifactsin the interaction pattern. An increased ability of multi-touchdetection of a touch-sensing apparatus may be exploited to improve theuser experience.

It should be realized that both of the actual interaction pattern andthe modified interaction pattern are approximations of a trueinteraction pattern on the surface portion, and that the modifiedinteraction pattern contains a deliberate and desired distortion aimedat enhancing certain features in the interaction pattern. The desireddistortion of the resulting interaction pattern is caused bymanipulating the input data to the reconstruction algorithm.

In one embodiment, the modified interaction pattern represents anenhancement of weakly interacting touches over strongly interactingtouches in the actual interaction pattern.

In one embodiment, each interaction pattern comprises a distribution ofinteraction values within at least part of the surface portion, whereineach of the interaction values indicates a local attenuation of energy.

In one embodiment, said set of data samples represents an actual degreeof interaction between one or more touches on the surface portion andthe detection lines, and the step of generating the modified set of datasamples comprises: actively modifying the actual degree of interactionfor at least part of the detection lines. The modifying may comprise:changing the mutual relation in the actual degree of interaction amongthe different detection lines, and/or relatively decreasing the actualdegree of interaction for the detection lines with the highest actualdegree of interaction.

In one embodiment, the step of generating the modified set of datasamples comprises: obtaining, based on the output signal, a magnitudevalue for each detection line; and applying a predetermined re-scalingfunction to the magnitude value for the detection lines. The re-scalingfunction may be non-linear and have a decreasing derivate withincreasing magnitude value, at least for non-negative magnitude values.Alternatively or additionally, the re-scaling function may be defined bya set of control parameters, and the method may comprise a step ofsetting at least one of the control parameters based on the magnitudevalues for the detection lines.

In one embodiment, the method further comprises: obtaining, based on theoutput signal, a magnitude value for each detection line; obtaining areference interaction pattern on the surface portion; identifying alocation of a strongly interacting touch in the reference interactionpattern; identifying a set of detection lines intersecting saidlocation; and actively modifying the magnitude values for the set ofdetection lines.

The step of generating the modified set of data samples may comprise atleast one of: changing the magnitude values for the set of intersectingdetection lines by a predetermined amount or fraction, setting themagnitude values for the set of intersecting detection lines to apredefined value according to a predefined criterion, and decreasing themagnitude values for the set of intersecting detection lines by anestimated contribution of the strongly interacting touches.Alternatively or additionally, the step of obtaining the referenceinteraction pattern may comprise: operating the reconstruction algorithmon the set of data samples so as to generate the actual interactionpattern, and obtaining the reference interaction pattern based on theactual interaction pattern. Alternatively or additionally, the step ofidentifying the set of intersecting detection lines may comprise:accessing a data structure that links regions on the surface portion tothe detection lines that intersect the regions. Alternatively oradditionally, each detection line may be defined by first and seconddimension values in a two-dimensional sample space, wherein the firstand second dimension values may define the location of the detectionline on the surface portion, and wherein the step of identifying the setof intersecting detection lines may comprise: mapping the location ofthe strongly interacting touch to a predetermined curve in the samplespace, and identifying the set of intersecting detection lines byintersecting the predetermined curve with the detection lines as mappedto the sample space.

In one embodiment, the magnitude values for the detection lines indicatea degree of interaction between one or more touches on the surfaceportion and the detection lines.

In one embodiment, the magnitude values for the detection linesrepresent the set of data samples. Thus, if provided in or converted tothe input format, the magnitude values would form the set of datasamples, which in turn would result in the actual interaction pattern.The deliberate modification of the magnitude values, according to thevarious embodiments of the invention, enables generation of the modifiedset of data samples and the modified interaction pattern.

In one embodiment, the method repeatedly executes a sequence of stepscomprising: the step of obtaining the output signal, the step ofgenerating the modified set of data samples, and the step of operatingthe reconstruction algorithm on the modified set of data samples, a stepof determining touch data based on the modified interaction pattern, anda step of outputting the touch data.

In one embodiment, the output signal represents detected signal energyon the respective detection lines.

In one embodiment, said input format represents a decrease in signalenergy caused by interaction between one or more touches on the surfaceportion and one of the detection lines.

In one embodiment, said input format is represented by a function ofdetected signal energy for the respective detection line normalized by areference value, wherein the reference value represents the detectedsignal energy on the respective detection line without touches on thesurface portion. The input format may be given by operating a logarithmfunction on the detected signal energy for the respective detection linenormalized by the reference value.

In one embodiment, the predetermined reconstruction algorithm isdesigned for tomographic reconstruction based on data in said inputformat.

The modified interaction pattern may be generated as a distorted versionof an actual interaction pattern that represents changes in interactionon any time scale. In one example, the actual interaction patternrepresents changes with respect to a clean touch surface withouttouches, such that all interaction essentially originates from a currentpresence of touches on the surface portion. In another example, theactual interaction pattern represents changes on a shorter time scale,e.g. interaction changes between a current execution of the method stepsand a preceding execution of the method steps.

A second aspect of the invention is a computer program productcomprising computer code which, when executed on a data-processingsystem, is adapted to carry out the method of the first aspect.

A third aspect of the invention is a device for enabling touchdetermination based on an output signal from a touch-sensitiveapparatus. The touch-sensitive apparatus comprises a panel configured toconduct signals from a plurality of incoupling points to a plurality ofoutcoupling points, thereby defining detection lines that extend acrossa surface portion of the panel between pairs of incoupling andoutcoupling points, signal generating means coupled to the incouplingpoints to generate the signals, and signal detecting means coupled tothe outcoupling points to generate the output signal which is indicativeof one or more touches present on the surface portion. The devicecomprises: means for obtaining the output signal which, if convertedinto a set of data samples of a given input format, enables apredetermined reconstruction algorithm to determine an actualinteraction pattern on said surface portion; means for generating, basedon the output signal, a modified set of data samples in said given inputformat; and means for operating the predetermined reconstructionalgorithm on the modified set of data samples so as to determine amodified interaction pattern on said surface portion.

A fourth aspect of the invention is a touch-sensitive apparatus,comprising: a panel configured to conduct signals from a plurality ofincoupling points to a plurality of outcoupling points, thereby definingdetection lines that extend across a surface portion of the panelbetween pairs of incoupling and outcoupling points; means for generatingthe signals at the incoupling points; means for generating an outputsignal based on detected signals at the outcoupling points, the outputsignal being indicative of one or more touches present on the surfaceportion; and the device according to the third aspect.

A fifth aspect of the invention is a touch-sensitive apparatus,comprising: a panel configured to conduct signals from a plurality ofincoupling points to a plurality of outcoupling points, thereby definingdetection lines that extend across a surface portion of the panelbetween pairs of incoupling and outcoupling points; at least one signalgenerator coupled to the incoupling points to generate the signals; atleast one signal detector coupled to the outcoupling points to generatean output signal which is indicative of one or more touches present onthe surface portion. The touch-sensitive apparatus further comprises asignal processor connected to receive the output signal and configuredto: obtain the output signal which, if converted into a set of datasamples of a given input format, enables a predetermined reconstructionalgorithm to determine an actual interaction pattern on said surfaceportion; generate, based on the output signal, a modified set of datasamples in said given input format; and operate the predeterminedreconstruction algorithm on the modified set of data samples so as todetermine a modified interaction pattern on said surface portion.

Any one of the embodiments of the first aspect can be combined with thesecond to fifth aspects.

Still other objectives, features, aspects and advantages of the presentinvention will appear from the following detailed description, from theattached claims as well as from the drawings.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the invention will now be described in more detail withreference to the accompanying schematic drawings.

FIG. 1 is a plan view of a touch-sensitive apparatus.

FIGS. 2A-2B are side and top plan views of touch-sensitive systemsoperating by frustrated total internal reflection (FTIR).

FIG. 3A is a flow chart of a reconstruction method, and FIG. 3B is ablock diagram of a device that implements the method of FIG. 3A.

FIG. 4A illustrates an actual interaction pattern obtained byreconstruction processing, and FIG. 4B illustrates the actualinteraction pattern in FIG. 4A after scaling to enhance weak signalfeatures.

FIG. 5 is a block diagram of a structure for implementing a firstembodiment of data processing.

FIG. 6A-6C are graphs of different re-scaling functions for use upstreamof the reconstruction algorithm in the first embodiment.

FIG. 7 illustrates a modified interaction pattern obtained by the firstembodiment of data processing.

FIG. 8 is a block diagram of a structure for implementing a secondembodiment of data processing.

FIG. 9 illustrates a modified interaction pattern obtained by the secondembodiment of data processing.

FIG. 10 is a sinogram obtained by mapping reconstruction values foravailable detection lines to a two-dimensional sample space.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

The present invention relates to techniques for enabling extraction oftouch data for multiple objects in contact with a touch surface of atouch-sensitive apparatus. The description starts by presenting theunderlying concept of such a touch-sensitive apparatus, especially anapparatus operating by frustrated total internal reflection (FTIR) oflight. The description continues to present embodiments for suppressingstrong touches in interaction patterns generated by processing ofsignals obtained from a touch-sensitive apparatus. Finally, detailedexamples are given.

Throughout the description, the same reference numerals are used toidentify corresponding elements.

1. Touch-Sensitive Apparatus

FIG. 1 illustrates a touch-sensitive apparatus 100 which is based on theconcept of transmitting energy of some form across a touch surface 1,such that an object that is brought into close vicinity of, or incontact with, the touch surface 1 causes a local decrease in thetransmitted energy. The touch-sensitive apparatus 100 includes anarrangement of emitters and sensors, which are distributed along theperiphery of the touch surface 1. Each pair of an emitter and a sensordefines a detection line, which corresponds to the propagation path foran emitted signal from the emitter to the sensor. In FIG. 1, only onesuch detection line D is illustrated to extend from emitter 2 to sensor3, although it should be understood that the arrangement typicallydefines a dense grid of intersecting detection lines, each correspondingto a signal being emitted by an emitter and detected by a sensor. Anyobject that touches the touch surface along the extent of the detectionline D will thus decrease its energy, as measured by the sensor 3. Thus,a touch on the touch surface 1 by an object results in an attenuation ofone or more detection lines.

The arrangement of sensors 3 is electrically connected to a signalprocessor 10, which samples and processes an output signal from thearrangement. The output signal is indicative of the received energy ateach sensor 3. As will be explained below, the signal processor 10 maybe configured to process the output signal so as to recreate an image ofthe distribution of interaction values (for simplicity, referred to asan “interaction pattern” or “attenuation field” in the following) acrossthe touch surface 1. The interaction pattern may be further processed bythe signal processor 10 or by a separate device (not shown) for touchdetermination, which may involve extraction of touch data, such as aposition (e.g. x, y coordinates), a shape or an area of each touchingobject.

In the example of FIG. 1, the touch-sensitive apparatus 100 alsoincludes a controller 12 which is connected to selectively control theactivation of the emitters 2 and, possibly, the readout of data from thesensors 3. The signal processor 10 and the controller 12 may beconfigured as separate units, or they may be incorporated in a singleunit. One or both of the signal processor 10 and the controller 12 maybe at least partially implemented by software executed by a processingunit.

Generally, the touch-sensitive apparatus 100 (the touch surface 1) maybe of any shape, such as circular, elliptical or polygonal, includingrectangular. The touch-sensitive apparatus 100 may be designed to beused with a display device or monitor, e.g. as described in theBackground section.

The touch-sensitive apparatus 100 may be configured to permittransmission of energy in one of many different forms. The emittedsignals may thus be any radiation or wave energy that can travel in andacross the touch surface 1 including, without limitation, light waves inthe visible or infrared or ultraviolet spectral regions, electricalenergy, electromagnetic or magnetic energy, or sonic and ultrasonicenergy or vibration energy.

In the following, an example embodiment based on propagation of lightwill be described. FIG. 2A is a side view of a touch-sensitive apparatus100 which includes a light transmissive panel 4, one or more lightemitters 2 (one shown) and one or more light sensors 3 (one shown). Thepanel 4 defines two opposite and generally parallel surfaces 5, 6 andmay be planar or curved. A radiation propagation channel is providedbetween two boundary surfaces 5, 6 of the panel 4, wherein at least oneof the boundary surfaces allows the propagating light to interact with atouching object 7. Typically, the light from the emitter(s) 2 propagatesby total internal reflection (TIR) in the radiation propagation channel,and the sensors 3 are arranged at the periphery of the panel 4 togenerate a respective measurement signal which is indicative of theenergy (or equivalently, the power or intensity) of received light.

As shown in FIG. 2A, the light may be coupled into and out of the panel4 directly via the edge portion that connects the top and bottomsurfaces 5, 6 of the panel 4. Alternatively, not shown, a separatecoupling element (e.g. in the shape of a wedge) may be attached to theedge portion or to the top or bottom surface 5, 6 of the panel 4 tocouple the light into and/or out of the panel 4. When the object 7 isbrought sufficiently close to the boundary surface, part of the lightmay be scattered by the object 7, part of the light may be absorbed bythe object 7, and part of the light may continue to propagate in thepanel 4. Thus, when the object 7 touches a boundary surface of the panel(e.g. the top surface 5), the total internal reflection is frustratedand the energy of the transmitted light is decreased. This type oftouch-sensitive apparatus is denoted “FTIR system” (FTIR—FrustratedTotal Internal Reflection) in the following.

The touch-sensitive apparatus 100 may be operated to measure the energyof the light transmitted through the panel 4 on a plurality of detectionlines. This may, e.g., be done by activating a set of spaced-apartemitters 2 to generate a corresponding number of light sheets inside thepanel 4, and by operating a set of sensors 3 to measure the transmittedenergy of each light sheet. Such an embodiment is illustrated in FIG.2B, where each emitter 2 generates a beam of light that expands in theplane of the panel 4 while propagating away from the emitter 2. Eachbeam propagates from one or more entry or incoupling points on the panel4. Arrays of light sensors 3 are located around the perimeter of thepanel 4 to receive the light from the emitters 2 at a number ofspaced-apart outcoupling points on the panel 4. It should be understoodthat the incoupling and outcoupling points merely refer to the positionwhere the beam enters and leaves, respectively, the panel 4. Thus, oneemitter/sensor may be optically coupled to a number ofincoupling/outcoupling points. In the example of FIG. 2B, however, thedetection lines D are defined by individual emitter-sensor pairs. Thisimplementation and further variants are disclosed in more detailed inApplicant's WO2010/064983, which is incorporated herein in its entiretyby this reference.

It is to be understood that FIG. 2 merely illustrates one example of anFTIR system. For example, the detection lines may instead be generatedby sweeping or scanning one or more beams of light inside the panel.Such and other examples of FTIR systems are e.g. disclosed in U.S. Pat.No. 6,972,753, U.S. Pat. No. 7,432,893, US2006/0114237, US2007/0075648,WO2009/048365, WO2010/006882, WO2010/006883, WO2010/006884,WO2010/006885, WO2010/006886, and WO2011/134865, which are allincorporated herein by this reference. The inventive concept may beadvantageously applied to such alternative FTIR systems as well.

Irrespective of implementation, the light sensors 3 collectively providean output signal, which is received and sampled by the signal processor10. The output signal contains a number of sub-signals, also denoted“projection signals”, each representing the energy of light emitted by acertain light emitter 2 and received by a certain light sensor 3, i.e.the received energy on a certain detection line. Depending onimplementation, the signal processor 10 may need to process the outputsignal for separation of the individual projection signals.

2. Modification of Data for Suppressing Strong Touches

In its various aspects, the invention relates to a touch determinationtechnique that is able to detect multiple touches on the touch surface,even if the touches have different degrees of interaction with thedetection lines, e.g. even if one or more touches result in asignificantly stronger attenuation of the propagating light in an FTIRsystem compared to one or more other touches. In various embodiments,this ability for improved touch determination is achieved by activemodification of a distribution of magnitude values obtained from theprojection signals, where each magnitude value directly or indirectlyrepresents the degree of interaction between the touches and arespective detection line.

As will be further explained below, the active modification may operatedirectly on the values (also denoted “projection values”) of theprojection signals, on values (also denoted “reconstruction values”) ofreconstruction signals which are obtained by processing the projectionsignals and which are adapted for input to a reconstructionalgorithm/process, or on values (also denoted “intermediate values”) ofany type of intermediate signals obtained by processing the projectionsignals.

FIG. 3A illustrates an embodiment of a method for reconstruction andtouch data extraction in a touch-sensitive apparatus, such as theabove-described FTIR system. In the illustrated embodiment, the activemodification operates on the above-mentioned reconstruction signals.

The method involves a sequence of steps 20-28 that are repeatedlyexecuted, typically by the signal processor 10 (FIGS. 1-2). In thecontext of this description, each sequence of steps 20-28 is denoted asensing instance.

Each sensing instance starts by a data collection step 20, in whichmeasurement values are obtained from the light sensors 3 in the FTIRsystem, typically by sampling a value from each of the aforesaidprojection signals. The data collection step 20 results in oneprojection value for each detection line. It may be noted that the datamay, but need not, be collected for all available detection lines in theFTIR system. The data collection step 20 may also include pre-processingof the measurement values, e.g. filtering for noise reduction.

In a conversion step 22, the projection values are processed forconversion into the above-mentioned reconstruction values, which aregiven in a format adapted to the reconstruction algorithm that is used(in the reconstruction step 26, below) for generating the interactionpattern. It should be emphasized that the format of the reconstructionvalues depends on (or is given by) the type of reconstruction algorithm,and the format typically represents the decrease in signal energy causedby the interaction between touches and detection lines. In the detailedexamples below, the format is given as the (negative) logarithm of thesignal transmission for the detection line, the signal transmissionbeing given by the projection value normalized by a reference value (seebelow). In alternative implementations, the format may be given as atransmission (e.g. given by the projection value normalized by thereference value), an attenuation (e.g. given by 1−transmission), anenergy difference (e.g. given by the difference between the projectionvalue and the reference value), or a logarithm of the attenuation or theenergy difference. As used hereinabove, a “logarithm” is intended toalso encompass functions approximating a true logarithmic function, inany base. Furthermore, all of the above conversion functions may haveany sign, i.e. they may or may not be multiplied by a negative value.

Depending on the reconstruction algorithm, it may be possible to omitthe conversion step 22, whereby the reconstruction values are equal tothe projection values.

In a modification step 24, the reconstruction values are processed tosuppress the influence of strong touches compared to weak touches. Thismay be seen as a process of changing the dynamics of the ensemble ofreconstruction values, i.e. the relation between the reconstructionvalues for different detection lines, so as to suppress stronglyinteracting touches in the interaction pattern to be reconstructed.Thus, the modification step 24 generates a modified set ofreconstruction values, by actively modifying the relative distributionof reconstruction values among the detection lines. As will be explainedfurther below with reference to detailed examples, this may be achievedin different ways, e.g. by applying a re-scaling function to thereconstruction values, or by identifying strongly interacting touches ina reference pattern and actively modifying the reconstruction valuesonly for those detection lines that are deemed affected by thesestrongly interacting touches. In the following, the modified set ofreconstruction values is also referred to as “modified reconstructionvalues”, although it is to be understood that only part of thereconstruction values may need to be modified.

In a reconstruction step 26, the interaction pattern on the touchsurface is reconstructed by processing the modified reconstructionvalues that are derived in the modification step 24. The interactionpattern is a distribution of interaction values across the touch surface(or a relevant part of the touch surface). Each interaction valuetypically represents a local attenuation of energy in a specificposition or reconstruction cell (pixel) on the touch surface.

Here, it is important to understand that the reconstruction algorithm isadapted to operate on reconstruction values in a specific format, whichmeans that the reconstruction values are obtained from the projectionvalues according to a predetermined function or algorithm. Thereconstruction values may, but need not, be correct (at least in anapproximation) in view of the physical model that underlies thereconstruction algorithm. The format fits to the reconstructionalgorithm to such an extent that the reconstruction algorithm, ifoperated on the reconstruction values, yields an interaction patternwhich is a better representation of the “true” interaction pattern thanthe interaction pattern obtained by operating the reconstructionalgorithm on the modified reconstruction values. Therefore, in thecontext of the present disclosure, the former interaction pattern isdenoted “actual interaction pattern” and the latter interaction patternis denoted “modified interaction pattern” or “distorted interactionpattern”. However, it should be understood that the actual interactionpattern may deviate, even significantly, from the true interactionpattern, e.g. as a result of measurement or processing noise, inaccuracyor non-linearity in signal detection, intentional or unintentionalsimplifications in the physical model and/or reconstruction algorithm,etc.

Any available reconstruction algorithm may be used in the reconstructionstep 26, including tomographic reconstruction methods such as FilteredBack Projection, FFT-based algorithms, ART (Algebraic ReconstructionTechnique), SART (Simultaneous Algebraic Reconstruction Technique), etc.Alternatively, the reconstruction algorithm may generate the interactionpattern by adapting one or more basis functions to the reconstructionvalues and/or by statistical methods such as Bayesian inversion.Examples of such reconstruction algorithms designed for use in touchdetermination are found in WO 2010/006883, WO2009/077962, WO2011/049511,WO2011/139213, PCT/SE2011/051201 filed on Oct. 7, 2011 and US61/552,024filed on Oct. 27, 2011, all of which are incorporated herein byreference. Conventional reconstruction methods are found in themathematical literature, e.g. “The Mathematics of ComputerizedTomography” by Natterer, and “Principles of Computerized TomographicImaging” by Kak and Slaney.

The interaction pattern may be reconstructed within one or more subareasof the touch surface. The subareas may be identified by analyzingintersections of detection lines across the touch surface, based on theabove-mentioned projection signals. Such a technique for identifyingsubareas is further disclosed in WO2011/049513, which is incorporatedherein by this reference.

In a subsequent extraction step 28, the modified interaction pattern isprocessed for identification of touch-related features and extraction oftouch data. Any known technique may be used for isolating touches withinthe modified interaction pattern. For example, ordinary blob detectionand tracking techniques may be used for finding the touches. In oneembodiment, a threshold is first applied to the modified interactionpattern, to remove noise. Any areas with interaction values that fallbelow or above (depending on implementation) the threshold may befurther processed to find the center and shape by fitting for instance atwo-dimensional second-order polynomial or a Gaussian bell shape to theinteraction values, or by finding the ellipse of inertia of theinteraction values. There are also numerous other techniques as is wellknown in the art, such as clustering algorithms, edge detectionalgorithms, standard blob detection, water shedding techniques, floodfill techniques etc.

Any available touch data may be extracted, including but not limited tox,y coordinates, areas, shapes and/or pressure of the touches.

After step 28, the extracted touch data is output, and the processreturns to the data collection step 20.

It is important to notice that several variations of the embodiment inFIG. 3A are possible. The aim of the modification step 24 is to producethe modified reconstruction values to be input to the reconstructionalgorithm in step 26. In a variant, the modification step 24 is includedbetween the data collection step 20 and the conversion step 22, so as tomodify the projection values before the conversion step 22, which thenmay directly generate the modified reconstruction values based on themodified projection values. In a similar variant, the modification step24 is included in the data collection step 20, which thereby producesthe modified set of projection values. In another variant, themodification step 24 is included in the conversion step 22, whichthereby directly generates the modified reconstruction values.Combinations of the above variants are also conceivable, i.e. that thetouch determination process includes plural modification steps thatcollectively operate to generate the modified reconstruction values.When the modification step is “included in” another step, themodification step is embedded among other functional operations and,structurally, the modification is not a separate step of the method.However, functionally, the method includes a modification step.

With reference to the embodiment of FIG. 3A, and variants thereof, it isto be understood that one or more of steps 20-28 may be effectedconcurrently. For example, the data collection step 20 of a subsequentsensing instance may be initiated concurrently with any one of steps22-28.

The method for reconstruction and touch data extraction may be executedby a data processing device (cf. signal processor 10 in FIGS. 1-2) whichis connected to obtain the measurement values from the light sensors 3in the FTIR system. FIG. 3B shows an example of such a data processingdevice 10 for implementing the method in FIG. 3A. In the illustratedexample, the device 10 includes an input 200 for receiving the outputsignal. The device 10 further includes a data collection element (ormeans) 202 for processing the output signal to generate the projectionvalues, a conversion element (or means) 204 for converting theprojection values into reconstruction values, a modification element (ormeans) 206 for modifying the reconstruction values, a reconstructionelement (or means) 208 for generating the modified interaction pattern,and an output 210 for outputting the modified interaction pattern. Inthe example of FIG. 3B, the actual extraction of touch data is carriedout by a separate device 10′ which is connected to receive the modifiedinteraction pattern from the data processing device 10.

The data processing device 10 may be implemented by special-purposesoftware (or firmware) run on one or more general-purpose orspecial-purpose computing devices. In this context, it is to beunderstood that each “element” or “means” of such a computing devicerefers to a conceptual equivalent of a method step; there is not alwaysa one-to-one correspondence between elements/means and particular piecesof hardware or software routines. One piece of hardware sometimescomprises different means/elements. For example, a processing unit mayserve as one element/means when executing one instruction, but serve asanother element/means when executing another instruction. In addition,one element/means may be implemented by one instruction in some cases,but by a plurality of instructions in some other cases. Naturally, it isconceivable that one or more elements (means) are implemented entirelyby analog hardware components.

The software controlled computing device may include one or moreprocessing units, e.g. a CPU (“Central Processing Unit”), a DSP(“Digital Signal Processor”), an ASIC (“Application-Specific IntegratedCircuit”), discrete analog and/or digital components, or some otherprogrammable logical device, such as an FPGA (“Field Programmable GateArray”). The data processing device 10 may further include a systemmemory and a system bus that couples various system components includingthe system memory to the processing unit. The system bus may be any ofseveral types of bus structures including a memory bus or memorycontroller, a peripheral bus, and a local bus using any of a variety ofbus architectures. The system memory may include computer storage mediain the form of volatile and/or non-volatile memory such as read onlymemory (ROM), random access memory (RAM) and flash memory. Thespecial-purpose software may be stored in the system memory, or on otherremovable/non-removable volatile/non-volatile computer storage mediawhich is included in or accessible to the computing device, such asmagnetic media, optical media, flash memory cards, digital tape, solidstate RAM, solid state ROM, etc. The data processing device 10 mayinclude one or more communication interfaces, such as a serialinterface, a parallel interface, a USB interface, a wireless interface,a network adapter, etc, as well as one or more data acquisition devices,such as an A/D converter. The special-purpose software may be providedto the data processing device 10 on any suitable computer-readablemedium, e.g. a record medium or a read-only memory.

3. Choice of Reconstruction Values

As noted above, the format of the reconstruction values is determined bythe type of reconstruction algorithm. Many tomographic reconstructionalgorithms are designed to reconstruct an attenuation field, i.e. eachinteraction value in the reconstructed interaction pattern represents alocal attenuation of energy by an attenuating medium.

Reverting to the FTIR system in FIG. 2A, the light will not be blockedby the touching object 7. Thus, if two objects 7 happen to be placedafter each other along a light path from an emitter 2 to a sensor 3,both objects 7 will interact with the propagating light. Provided thatthe light energy is sufficient, a remainder of the light will reach thesensor 3 and generate an output signal that allows both interactions(touches) to be identified. Thus, in multi-touch FTIR systems, thetransmitted light may carry information about a plurality of touches.

In the following, T_(k) is the transmission for the k:th detection lineD_(k), T_(v) is the transmission at a specific position along thedetection line D_(k), and A_(v) is the relative attenuation at thespecific position. The total transmission (modeled) along a detectionline is thus:

$T_{k} = {{\prod\limits_{v}\; T_{v}} = {\prod\limits_{v}\;( {1 - A_{v}} )}}$

The above equation is suitable for analyzing the attenuation caused bydiscrete objects on the touch surface, when the points are fairly largeand separated by a distance. However, a more correct definition ofattenuation through an attenuating medium may be used:I _(k) =I _(0,k)·(e ^(−∫a(x)dx))→T _(k) =I _(k) /I _(0,k) =e ^(−∫a(x)dx)

In this formulation, I_(k) represents the transmitted energy ondetection line D_(k) with attenuating object(s), I_(0,k) represents thetransmitted energy on detection line D_(k) without attenuating objects,and a(x) is the attenuation coefficient along the detection line D_(k).In this formulation, the detection line is assumed to interact with thetouch surface along the entire extent of the detection line, i.e. thedetection line is represented as a mathematical line.

It is thus realized that a tomographic reconstruction model may bedesigned to operate on transmission data for the detection lines. Suchtransmission data may be obtained by dividing the projection values by arespective background or reference value. By proper choice of backgroundvalues, the projection values are thereby converted into transmissionvalues, which thus represent the fraction (e.g. in the range [0, 1]) ofthe available light energy that has been measured on each of thedetection lines. The background signal may be pre-set (e.g. by factorycalibration), derived during a separate calibration step before or afterthe touch determination process, or derived from the projection valuesacquired during one or more preceding sensing instances without anyobjects touching the panel, possibly by averaging a set of suchprojection values. For more advanced techniques of generating thebackground values, in which the background values are selectivelyupdated by analyzing the projection values obtained during the touchdetermination process (cf. FIG. 3A), reference is made to WO2011/028169and WO2011/049512, which are incorporated herein by reference.

Certain tomographic reconstruction techniques, such as Filtered BackProjection (FBP) are based on the theory of the Radon transform whichdeals with line integrals. Such reconstruction techniques may thereforebe designed to operate on reconstruction values d_(k) that are given bythe negative logarithm of the transmission:d _(k)=−log(T _(k))=−log(e ^(−∫a(x)dx))=∫∫a(x)dx

In a variant, the reconstruction values d_(k) may be given by any knownapproximation of this conversion function. A simple approximation of−log(T_(k)), which is a good approximation for T_(k) close to 1 and maybe useful also for smaller values of T_(k), is given by d_(k)=1−T_(k).

4. Detailed Examples

In this section, two main approaches to generate modified reconstructionvalues will be exemplified in greater detail.

The performance improvement of the respective approach is illustrated bycomparison to an interaction pattern which is obtained by operating atomographic reconstruction algorithm on reconstruction values obtainedby converting a given set of projection values via −log(T_(k)). Theprojection values represent received light energy in an FTIR system ofthe type shown in FIG. 2B. FIG. 4A illustrates the resulting (actual)interaction pattern in the coordinate system of touch surface, wheredark areas indicate high attenuation. FIG. 4A has been generated byautoscaling the interaction pattern between minimum and maximum values.From FIG. 4A, it appears as if there are only two touches in the actualinteraction pattern. However, the set of projection signals are actuallygenerated with four objects on the touch surface, but with two of theobjects resulting in a significantly weaker interaction (a factor of 10)than the other two objects. In FIG. 4B, the interaction pattern in FIG.4A has been re-scaled to enhance weak features. Here, the two weaklyinteracting touches may be discerned in between the strongly interactingtouches. It is seen that the interaction values of the weaklyinteracting touches are partially concealed by artifacts caused by thestrongly interacting touches. In the example of FIG. 4B, the artifactsinclude star-like streaks emanating from the strongly interactingtouches. For example, artifacts may originate from a local deformationof the touch surface caused by the touching objects, and/or they mayresult from an inability of the reconstruction algorithm to accuratelyreproduce the true interaction pattern.

To further explain the origin of artifacts, assume that thereconstruction values, d, depend on an attenuation field, a, on thetouch surface according to a projection function

, which reflects the properties of the physical touch system:d=

(a).

The reconstruction step (cf. step 26 in FIG. 3A) is aimed atreconstructing the attenuation field a₀ (an approximation of the trueinteraction pattern a) from the reconstruction values d, using areconstruction function

:a ₀=

(d).

Typically, the reconstruction function

is not the exact inverse of the projection function

. One reason may be that certain properties of the physical touch systemmay be difficult and/or computationally expensive to include in

. Another reason may be that

is based on mathematical principles that do not allow a perfectreconstruction. In either case, the reconstructed interaction pattern a₀will contain artifacts. Typically, strong touches will introducestronger artifacts than weak ones, and this is certainly the case if

is a linear function of d. If the touch system is exposed to touchesthat differ significantly in interaction strength, even by one orseveral orders of magnitude, there is a significant risk that artifactsfrom a strongly interacting touch might conceal a weakly interactingtouch. Furthermore, the presence of significant noise levels may makeidentification of weakly interacting touches even more difficult.

4.1. Use of a Re-Scaling Function

One main approach to enhance weakly interacting touches over stronglyinteracting touches in the interaction pattern, is to apply a re-scalingfunction that reduces the dynamics between strong and weak touches inthe reconstruction values that are fed to the reconstruction algorithm.As noted above, the re-scaling function may be applied to modify thereconstruction values, the projection values, or some intermediatevalues used in the touch determination process (cf. FIG. 3A). In thefollowing examples, it is assumed that the re-scaling function isapplied to modify the reconstruction values of the different detectionlines.

FIG. 5 is a schematic block diagram of a processing structure fordetermining touch data that implements such a re-scaling function. Theprocessing structure comprises a block 51 which obtains the outputsignal from the light sensors 3 and generates a set d of reconstructionvalues for the different detection lines. A block 52 applies there-scaling function to the projection values to generate a modified setd′ of reconstruction values for the detection lines. A block 53 operatesa reconstruction algorithm on the modified reconstruction values togenerate a modified interaction pattern a₁. A block 54 operates on themodified interaction pattern a₁ to determine touch data, which is outputfor downstream processing or other use.

In this example, the re-scaling function, ƒ, is applied to eachindividual reconstruction value before doing the reconstruction:a ₁=

(ƒ(d)).

Here ƒ is a function which applies the re-scaling function ƒelement-wise to the reconstruction values in d, to generate the modifiedreconstruction values d′. The re-scaling function ƒ will typically be anon-linear monotonically increasing function with a decreasing rate ofincrease, i.e. the derivative of the function decreases with increasingreconstruction value.

FIGS. 6A-6C illustrate three examples of suitable re-scaling functionsƒ. FIG. 6A is a graph of a logarithm-based function,ƒ_(l)(d_(k))=δ·log(1+γd_(k)), FIG. 6B is a graph of a re-scalingfunction based on the inverse hyperbolic sine function, ƒ_(a)(d_(k))=δ·asin h(γd_(k)), and FIG. 6C is a graph of a re-scaling function based ona sigmoid function (the logistic function),

${f_{s}( d_{k} )} = {\delta \cdot {( {\frac{2}{1 + {\mathbb{e}}^{{- \gamma}\; d_{k}}} - 1} ).}}$All of these re-scaling functions are approximately linear close tod_(k)=0, and the parameter γ controls how soon (when d_(k) increasesfrom 0) the re-scaling function becomes significantly non-linear withthe effect of scaling down large values of d_(k). In the graphs of FIGS.6A-6C, the control parameter γ was set to 10, 1000 and 10, respectively.The parameter δ may be used to scale the function linearly, for exampleso that the maximum reconstruction value is 1, if this is required bythe reconstruction algorithm. It should be noted that all of thesere-scaling functions may handle negative signal values, which may occurdue to noise, but that ƒ_(l) (FIG. 6A) is undefined for larger negativevalues and needs to handle such cases separately. Another approach, forall re-scaling functions, would be to set ƒ(d_(k))=0 when d_(k)<0.

The above re-scaling functions are only given as examples, and theskilled person immediately realizes that many other viable choices for ƒexist.

FIG. 7 illustrates the modified interaction pattern that is obtained bymodifying the reconstruction values d that was processed for generationof the interaction pattern in FIG. 4A, where the reconstruction valuesare modified by applying ƒ(d_(k))=δ·a sin h(γd_(k)), with γ=10⁷. Themodified interaction pattern in FIG. 7 has been generated by autoscalingthe interaction pattern between minimum and maximum values. Compared tothe actual interaction pattern in FIG. 4A, it is evident that bothweakly and strongly interacting touches are readily identified in themodified interaction pattern, irrespective of noise and reconstructionartifacts.

It should be noted that the touch determination process no longer aimsat reconstructing a close approximation of the true interaction pattern.This should be evident from the fact that the ratio between strongly andweakly interacting touches in the modified interaction pattern is likelyto deviate significantly from the true ratio. In many situations, thisis not a serious limitation, e.g. if the main task is to detect alltouches and estimate their locations with high enough accuracy. If theabsolute attenuation of touches is also required, this may be obtainedin several ways. One way to reconstruct the actual attenuation of atouch would be to locally reconstruct the actual interaction pattern, bycomputing

(d), at the required locations (e.g. inside the touch). If the function

produces significant artifacts from stronger touches, a more accurate(with less artifacts) but more time-consuming reconstruction function

may be applied to locally reconstruct the actual interaction pattern atthe required locations. Another way would be to use the iterativecompensation approach, described below, to suppress the contributionfrom all other touches in the reconstruction values d before doing thereconstruction. Yet another way would be to apply a compensatingfunction, ƒ′, to the modified interaction pattern, to counteract theeffect of the re-scaling function ƒ:a′ ₁=ƒ(

(ƒ(d))).

It should be noted however, that in general the application of anon-linear re-scaling function ƒ results in a distortion of thereconstruction

(ƒ(d)) which cannot be fully compensated by ƒ′.

As noted in connection with the exemplifying re-scaling functions inFIGS. 6A-6C, the re-scaling function ƒ may have a parameter γ thatcontrols which signal values are mapped approximately linearly and whichare mapped non-linearly (scaled down). Such a control parameter γ may beset depending on the range of signal levels produced by strong and heavytouches, and/or the noise level. Typically, it may be a good idea tohave the signal levels from the weakest touches in the (approximately)linearly mapped range, and stronger touches in the (significantly)non-linearly mapped range. Depending on implementation, the “signalvalues” may be projection values, reconstruction values or intermediatevalues.

4.2 Use of Iterative Compensation

Another main approach to enhance weakly interacting touches overstrongly interacting touches in the interaction pattern, is to obtain areference pattern on the touch surface and to identify the locations ofall strongly interacting touches in the reference pattern. Then, thedetection lines intersecting these locations are identified, and theprojection values/intermediate values/reconstruction values for thesedetection lines are modified to reduce the contribution from thestrongly interacting touches. In the following examples, it is assumedthat modification is done on the reconstruction values for theintersecting detection lines.

The reference pattern should thus allow the locations of the stronglyinteracting touches to be determined. In the following examples, thereference pattern is obtained by operating the reconstruction algorithmon the reconstruction values d obtained from the projection values:a ₀=

(d).

Thus, in a first reconstruction iteration, the actual interactionpattern a₀ is obtained for use as a reference pattern. As shown in FIG.4A, the actual interaction pattern readily allows strong touches to beidentified. Then, the modified reconstruction values d′ are determinedby suppressing or even removing, in the reconstruction values d, thecontribution from the strong touches that were identified in the actualinteraction pattern a₀. Thereafter, in a second reconstructioniteration, the reconstruction algorithm is operated on the modifiedreconstruction values d′ to generate a modified interaction pattern, a₁=

(d′), in which weakly interacting touches are enhanced over stronglyinteracting touches.

FIG. 8 is a schematic block diagram of a processing structure fordetermining touch data using this iterative compensation approach. Theprocessing structure comprises a block 81 which obtains the outputsignal from the light sensors 3 and generates the reconstruction valuesd for the different detection lines. A block 82, which is designed tocontrol the iterative compensation, first operates the reconstructionalgorithm on the reconstruction values d to generate the actualinteraction pattern a₀. A block 83 analyzes the actual interactionpattern a₀ to identify all strong touches, and the detection linesaffected by these strong touches. In this context “strong touches” maybe defined as all touches that are identifiable in the actualinteraction pattern a₀. Information p about the affected detectionlines, and possibly a contribution value for each such detection line,is output by block 83 for use by block 81. Block 81 is configured to usethe information p from block 83 to generate the modified reconstructionvalues d′, whereupon block 82 operates the reconstruction algorithm onthe modified reconstruction values d′ to generate the modifiedinteraction pattern a₁. A block 84 operates on the modified interactionpattern a₁, and possibly on the actual interaction pattern a₀, todetermine touch data which is output for downstream processing or otheruse.

In one embodiment, the modified reconstruction values d′ are generatedby decreasing the reconstruction values for the affected detection linesby an estimated contribution of the strong touches to the respectivereconstruction value (e.g. using the above-mentioned contributionvalue).

In an alternative embodiment, which requires less processing and whichmay be more robust, the modified reconstruction values d′ are generatedby setting the reconstruction values of the affected detection lines toa value that indicates a reduced interaction, e.g. by reducing thesereconstruction values by a predetermined amount or percentage (fraction)or setting them to a predefined value. In one embodiment, the predefinedvalue is selected such that the strong touches are essentiallyeliminated (“annihilated”) in the subsequently modified interactionpattern. For example, the predefined value may be set to indicate thatthe affected detection lines are unaffected by touches altogether. Ifthe reconstruction values are given by d_(k)=−log(T_(k)), the predefinedvalue may be zero (0).

By using such a predefined criterion, information about the weak touchesis also eliminated from the reconstruction values d, but in generalenough information from the weaker touches remains in the modifiedreconstruction values d′ to make the weaker touches detectable in themodified interaction pattern a₁ and allow good enough extraction oftouch data.

FIG. 9 illustrates the modified interaction pattern that is obtained bymodifying the reconstruction values d that were processed for generationof the interaction pattern in FIG. 4A. In this example, the modifiedreconstruction values d′ was obtained by setting all reconstructionvalues for the detection lines affected by the strong touches in FIG. 4Ato zero. The modified interaction pattern in FIG. 9 has been generatedby autoscaling the interaction pattern between minimum and maximumvalues. It is evident that the strongly interacting touches in theactual interaction pattern of FIG. 4A are missing, whereas both of theweakly interacting touches are readily identified in the modifiedinteraction pattern. Thus, by having access to both the actualinteraction pattern and the modified interaction pattern, block 84 (FIG.8) is able to determine touch data for all objects on the touch surface.

It is to be understood that the iterative compensation approach may beused with any reconstruction algorithm

, and that it would be possible to configure the processing structure toperform further iterations, i.e. to generate further modifiedreconstruction values d″ by removing the contribution from touchesdetected in the modified interaction pattern a₁, and to operate thereconstruction algorithm on the further modified reconstruction valuesd″ to generate a further modified interaction pattern a₂, etc.

It is conceivable that the actual interaction pattern is pre-processedbefore being used as the reference pattern for identifying the detectionlines that are affected by strong touches, for example for featureenhancement, noise reduction etc. In an alternative, the referencepattern is obtained from an actual interaction pattern determined in apreceding sensing instance.

Reverting to block 83 in FIG. 8, the strong touches may be identified byany known feature extraction technique, e.g. as discussed in relation tothe extraction step 28 in FIG. 3A. Once the strong touches areidentified, different techniques may be utilized to determine thedetection lines that are affected by the strong touches.

In one embodiment, block 83 has access to a data structure DB (FIG. 8)that links regions on the touch surface to the detection lines thatintersect the regions. For example, if the interaction pattern definesinteraction values in a grid of reconstruction cells (pixels) on thetouch surface, the data structure DB may associate each reconstructioncell, which has a certain position in the coordinate system of the touchsurface, to a set of intersecting detection lines. Thereby, block 83 isable to map the reconstruction cells included in each strong touch tothe data structure DB to identify the affected detection lines.

In another embodiment, each detection line is defined by first andsecond dimension values in a two-dimensional sample space, such that thefirst and second dimension values define the location of the detectionline on the touch surface. For example, in the realm of Filtered BackProjection (FBP), it is not uncommon to define detection lines in termsof a rotation angle φ of the detection line with respect to a referencedirection, and a distance s of the detection line from a predeterminedorigin. It is also well-known that the reconstruction values may bemapped to a two-dimensional sample space, a (φ, s) plane, so as to forma so-called sinogram. An example of such a sinogram is shown in FIG. 10,in which each square represents a reconstruction value of a detectionline. Each location on the touch surface corresponds to a predeterminedsinus curve in the (φ, s) plane. FIG. 10 illustrates three such curvesP1-P3 that correspond to three strong touches identified in an actualinteraction pattern. In fact, each illustrated curve P1-P3 is composedof several curves for each reconstruction cell covered by the respectivestrong touch in the actual interaction pattern. Furthermore, each suchcurve is given a certain width in the s direction, so as to form a bandin the (φ, s) plane. The width may be set in dependence of the size ofthe reconstruction cells in the interaction pattern.

It is realized that the affected detection lines may be identified bysimply mapping the location of each strong touch in the interactionpattern to its corresponding curve P1-P3 in the (φ, s) plane, and byintersecting each curve P1-P3 with the detection lines as mapped to the(φ, s) plane. If the reconstruction values are mapped to form a sinogramin the (φ, s) plane, as shown in FIG. 10, the predetermined criterionmay be applied at the same time as the mapping, e.g. by setting allreconstruction values intersected by the curves P1-P3 to a predeterminedvalue, e.g. zero.

For comparison, FIG. 10 also includes a curve P4 (dotted line) thatcorresponds to a weak touch, which is not detectable in actualinteraction pattern. It is realized that even if the reconstructionvalues of all affected detection lines are set to zero, a significantamount of reconstruction values remains along curve P4 enabling thereconstruction function to generate a modified interaction pattern inwhich the weak touch is detectable.

It should be realized that there are other parameter representations ofthe detection lines that may be used to define the sample space. Forexample, detection lines may be represented in a (β, α) plane, as isused in a fan geometry which is a standard geometry widely used inconventional tomography e.g. in the medical field. In such a standardgeometry, the detection lines may be defined in terms of an angularlocation β of the incoupling or outcoupling point of the detection linewith respect to a reference direction, and a rotation angle α of thedetection line. Alternatively, the incoupling points and the outcouplingpoints may be given by respective unique indexes, whereby a firstdimension of the sample space is given by an index of the incouplingpoints, and the second dimension of the sample space is given by anindex of the outcoupling points.

5. Concluding Remarks

The invention has mainly been described above with reference to a fewembodiments. However, as is readily appreciated by a person skilled inthe art, other embodiments than the ones disclosed above are equallypossible within the scope and spirit of the invention, which is definedand limited only by the appended patent claims.

It is to be understood that the modified interaction pattern, and theactual interaction pattern if applicable, may be subjected topost-processing before touch data extraction (cf. step 28 in FIG. 3A).Such post-processing may involve different types of filtering, e.g. fornoise removal and/or image enhancement.

The person skilled in the art realizes that there are other ways ofgenerating reconstruction values based on the output signal. Forexample, each individual projection signal included in the output signalmay be subjected to a high-pass filtering in the time domain, wherebythe thus-filtered projection signals represent background-compensatedenergy and may be processed for generation of reconstruction values.

The interaction pattern may be generated to represent changes ininteraction on the touch surface on any time scale. The foregoingexamples result in a modified interaction pattern which is a distortedversion of an actual interaction pattern that represents the currentinteraction on the touch surface, corresponding to changes ininteraction on a long time scale, e.g. since startup or calibration ofthe touch-sensitive apparatus. In an alternative, the modifiedinteraction pattern is generated as a distorted version of an actualinteraction pattern that represents changes in interaction on a shortertime scale, e.g. in the approximate range of 1 ms-100 s or 5 ms-5 s.Furthermore, all the above embodiments, examples, variants andalternatives given with respect to an FTIR system are equally applicableto a touch-sensitive apparatus that operates by transmission of otherenergy than light. In one example, the touch surface may be implementedas an electrically conductive panel, the emitters and sensors may beelectrodes that couple electric currents into and out of the panel, andthe output signal may be indicative of the resistance/impedance of thepanel on the individual detection lines. In another example, the touchsurface may include a material acting as a dielectric, the emitters andsensors may be electrodes, and the output signal may be indicative ofthe capacitance of the panel on the individual detection lines. In yetanother example, the touch surface may include a material acting as avibration conducting medium, the emitters may be vibration generators(e.g. acoustic or piezoelectric transducers), and the sensors may bevibration sensors (e.g. acoustic or piezoelectric sensors).

The invention claimed is:
 1. A method of enabling touch determinationbased on an output signal from a touch-sensitive apparatus, thetouch-sensitive apparatus comprising a panel configured to conductsignals from a plurality of incoupling points to a plurality ofoutcoupling points, thereby defining detection lines that extend acrossa surface portion of the panel between pairs of incoupling andoutcoupling points, at least one signal generator coupled to theincoupling points to generate the signals, and at least one signaldetector coupled to the outcoupling points to generate the outputsignal, the output signal being indicative of one or more touches on thesurface portion, wherein the method comprises: obtaining the outputsignal which, if converted into a set of data samples of a given inputformat, enables a reconstruction algorithm to determine an actualinteraction pattern on said surface portion; generating, based on theoutput signal, a modified set of data samples in said given inputformat; and operating the reconstruction algorithm on the modified setof data samples so as to determine a modified interaction pattern onsaid surface portion; wherein the modified interaction patternrepresents an enhancement of weakly interacting touches over stronglyinteracting touches in the actual interaction pattern.
 2. The method ofclaim 1, wherein each interaction pattern comprises a distribution ofinteraction values within at least part of the surface portion; and eachof the interaction values indicates a local attenuation of energy. 3.The method of claim 1, wherein said set of data samples represents anactual degree of interaction between the one or more touches on thesurface portion and the detection lines; and the generating the modifiedset of data samples includes actively modifying the actual degree ofinteraction for at least part of the detection lines.
 4. The method ofclaim 3, wherein said actively modifying comprises: changing a mutualrelation in the actual degree of interaction among different detectionlines.
 5. The method of claim 3, wherein said actively modifyingcomprises: relatively decreasing the actual degree of interaction forthe detection lines with the highest actual degree of interaction. 6.The method of claim 1, wherein said generating the modified set of datasamples comprises: obtaining, based on the output signal, a magnitudevalue for each detection line; and applying a re-scaling function to themagnitude values for the detection lines.
 7. The method of claim 6,wherein the re-scaling function is non-linear and has a decreasingderivate with increasing magnitude value, at least for non-negativemagnitude values.
 8. The method of claim 6, wherein the re-scalingfunction is defined by a set of control parameters; and the methodfurther includes setting at least one of the control parameters based onthe magnitude values for the detection lines.
 9. The method of claim 1,further comprising: obtaining, based on the output signal, a magnitudevalue for each detection line; obtaining a reference interaction patternon the surface portion; identifying a location of a strongly interactingtouch in the reference interaction pattern; identifying a set ofdetection lines intersecting said location; and actively modifying themagnitude values for the set of detection lines.
 10. The method of claim9, wherein the generating the modified set of data samples comprises atleast one of: changing the magnitude values for the set of detectionlines; setting the magnitude values for the set of detection lines to apredefined value according to a predefined criterion; and decreasing themagnitude values for the set of detection lines by an estimatedcontribution of the strongly interacting touches.
 11. The method ofclaim 9, wherein the obtaining the reference interaction patterncomprises: operating the reconstruction algorithm on the set of datasamples so as to generate the actual interaction pattern; and obtainingthe reference interaction pattern based on the actual interactionpattern.
 12. The method of claim 9, wherein the identifying the set ofdetection lines comprises: accessing a data structure that links regionson the surface portion to the detection lines that intersect theregions.
 13. The method of claim 9, wherein each detection line isdefined by first and second dimension values in a two-dimensional samplespace; the first and second dimension values define the location of thedetection line on the surface portion; and the identifying the set ofdetection lines includes mapping the location of the stronglyinteracting touch to a curve in the two-dimensional sample space, andidentifying the set of intersecting detection lines by intersecting thecurve with the detection lines as mapped to the two-dimensional samplespace.
 14. The method of claim 6, wherein the magnitude values for thedetection lines indicate a degree of interaction between the one or moretouches on the surface portion and the detection lines.
 15. The methodof claim 6, wherein the magnitude values for the detection linesrepresent the set of data samples.
 16. The method of claim 1, furthercomprising: repeatedly executing the obtaining the output signal, thegenerating the modified set of data samples, the operating thereconstruction algorithm on the modified set of data samples,determining touch data based on the modified interaction pattern, andoutputting the touch data.
 17. The method of claim 1, wherein the outputsignal represents detected signal energy on respective detection lines.18. The method of claim 1, wherein said input format represents adecrease in signal energy caused by interaction between one or more ofthe touches on the surface portion and one of the detection lines. 19.The method of claim 1, wherein said input format is represented by afunction of detected signal energy for a respective detection linenormalized by a reference value; and the reference value represents thedetected signal energy on the respective detection line without toucheson the surface portion.
 20. The method of claim 19, wherein said inputformat is given by operating a logarithm function on the detected signalenergy for the respective detection line normalized by the referencevalue.
 21. The method of claim 1, wherein the reconstruction algorithmis designed for tomographic reconstruction based on data in said inputformat.
 22. A non-transitory computer readable medium comprisingcomputer code which, when executed on a data-processing system, isadapted to carry out the method of claim
 1. 23. A device for enablingtouch determination based on an output signal from a touch-sensitiveapparatus, the touch-sensitive apparatus comprising a panel configuredto conduct signals from a plurality of incoupling points to a pluralityof outcoupling points, thereby defining detection lines that extendacross a surface portion of the panel between pairs of incoupling andoutcoupling points, at least one signal generator coupled to theincoupling points to generate the signals, and at least one signaldetector coupled to the outcoupling points to generate the outputsignal, the output signal being indicative of one or more touches on thesurface portion, said device comprising a signal processor configuredto: obtain the output signal which, if converted into a set of datasamples of a given input format, enables a reconstruction algorithm todetermine an actual interaction pattern on said surface portion;generate, based on the output signal, a modified set of data samples insaid given input format; and operate the reconstruction algorithm on themodified set of data samples to determine a modified interaction patternon said surface portion; wherein the modified interaction patternrepresents an enhancement of weakly interacting touches over stronglyinteracting touches in the actual interaction pattern.
 24. Atouch-sensitive apparatus, comprising: a panel configured to conductsignals from a plurality of incoupling points to a plurality ofoutcoupling points, thereby defining detection lines that extend acrossa surface portion of the panel between pairs of incoupling andoutcoupling points; at least one signal generator coupled to theincoupling points to generate the signals; at least one signal detectorcoupled to the outcoupling points to generate an output signal, theoutput signal being indicative of one or more touches on the surfaceportion; and a signal processor connected to receive the output signaland configured to obtain the output signal which, if converted into aset of data samples of a given input format, enables a reconstructionalgorithm to determine an actual interaction pattern on said surfaceportion, generate, based on the output signal, a modified set of datasamples in said given input format, and operate the reconstructionalgorithm on the modified set of data samples so as to determine amodified interaction pattern on said surface portion, wherein themodified interaction pattern represents an enhancement of weaklyinteracting touches over strongly interacting touches in the actualinteraction pattern.